Use of cold-formable austenitic stainless steel for valves for internal-combustion engines



teb. 7, 1967 E. J. DULIS ETAL 3,303,023

USE OF COLDFORMABLE AUSTENITIC STAINLESS STEEL FOR VALVES FOR INTERNAL-COMBUSTION ENGINES Filed Aug. 11, 1964 ml/W Carbon Up to 0.08 Chromium l6 f0 25 Nickel 8 to 20 Manganese Up to 3 Copper Up to 3 Nitrogen Up to 0.05 Carbon 8 Nitrogen Up to 0.08 Columbia/m 0.0/ 10 0./5

Baron 0. 0005 to 0.025

Silicon Up to 0.25

Iran 8 lmpurifies Remainder ml ll Agent United States Patent siguors to Crucible Steel Company of America, Pittsburgh, Pro, a corporation of New Jersey Filed Aug. 11, 1964, Ser. No. 389,524 2 Claims. ((11. 75-128) T his application is a continuation-in-part of our earlier filed copending application, Ser. No. 304,310 filed August 26, 1963, and now abandoned.

This invention relates to chromium-nickel stainless steels that are substantially completely and stably austenitic, and more particularly to such steels which are readily formable at ambient temperature but exhibit high resistance to deformation at elevated temperature and high resistance to the corrosive attack of various media at elevated temperatures. The outstanding high-temperature strength and corrosion resistance of our steels make them particularly well suited for the production of valves for use in internalcombustion engines and for the production of other vehicle-engine components. The superior cold-formability of our steels makes them useful for the manufacture of valves and similar parts by extrusion and/or forging at ambient temperature.

Although the prior art teaches other chromium-nickel steels which are substantially completely austenitic and exhibit the corrosion resistance and high elevated-temperature creep strength required of steels for use in making valves for internal-combustion engines, none hitherto known have, to our knowledge, been amenable to room temperature forging or extrusion. As a result, it has hitherto been necessary to restort to hot forming in the manufacture of such parts, with a resultant detrimental effect upon the surface quality and size uniformity of the hot-formed part. This has necessitated the use of further finishing process steps (machining, grinding, cleaning, etc.) that may be avoided, in large measure, if not entirely, by use of our steels. In the manufacture of engine valves and similar parts, it is sometimes desired to affix a hardened metal tip to the valve body or other part by welding, and it is another desirable attribute of a preferred steel of our invention that it possess, in combination with the properties indicated above, suitable weldablity.

Accordingly, it is an object of our invention to provide a substantially completely austenitic stainless steel exhibiting a desirable combination of high elevated-temperature creep strength and high resistance to corrosive attack of various media, including lead-containing fuel combustion properties indicated above, suitable weldability.

It is another object of our invention to provide substantially completely .austenitic stainless steel having relatively low ambient-temperature yield and tensile strengths and consequently being amenable to Working, forging, or extrusion at ambient temperature without the occurrence of stress cracking.

Still another object of our invention is to provide a steel exhibiting, in combination with the above-mentioned properties, desirable weldability.

Other objects of our invention in part will be obvious and in part pointed out more fully hereinafter.

The single figure of the accompanying drawings represents a specific product, and the steel composition thereof, falling within the scope of our invention.

In brief summary, we have discovered that steel possessing the above-indicated combination of desirable properties may be produced by providing a strengthened, substantially completely and stably austenitic chromium-nickel steel having low contents of carbon, nitrogen, and silicon. More particularly, we provide steel containing up to 0.08% of carbon, up to 0.05% of nitrogen, with the sum of the carbon and the nitrogen contents being up to 0.08%, and up to 0.25%, but preferably not over about 0.12%, of silicon. Our steel is strengthened to provide high-temperature creep strength substantially greater than that obtainable in a completely and stably aus-tenitic chromium-nickel steel to which no strengthening elements have been added, and desirably, the strengthening in our steel is done by the addition of small amounts of both columbium and boron, such as 0.01 to 0.15% columbium and 0.005 to 0.025% boron. The steel also contains at least 13%, preferably at least 16%, of chromium, and sufficient nickel to render it stably austenitic, the balance being iron except for incidental impurities. It is believed that the columbium and boron contribute to the creep strength (elevated-temperature deformation resistance) of the steel, while the low contents of silicon and carbon plus nitrogen contribute to the cold formability of the steel, neither of these modifications having any substantial detrimental effect upon the corrosion resistance of the steel. Moreover, the boron content of the steel is preferably maintained low, i.e., not over about 0.009% or 0.01%, to avoid detrimental effects upon both the weldability and the cold formability of the steel. In this manner, a steel possessing the above-indicated desirable combination of properties is obtained.

The steels of our invention may be readily distinguished from known commercial steels and those disclosed in the prior art in certain important respects. Commercial chromium-nickel austenitic stainless steels-are customarily melted to a silicon specification of 1.00% maximum. It is, more-over, the customary practice to deoxidize or kill austenitic stainless steel by additions to the mold of appropriate active silicon compounds. Between this consideration and the consideration that customary scrapselection and steelmaking practices introduce, and fail to eliminate, considerable quantities of silicon in austentic stainless steels, it is fair to state that commercal austenitic stainless steels rarely contain less than about 0.35% of silicon. The steels of our invention are further distinguishable from prior-art and commercial austenitic stainless steels in their containing an effective, but small, amount of columbium. In commercial austenitic stainless steels, columbium is usually used, when added, in an amount of at least ten times the carbon content, as an agent to precipitate the carbon remaining in the steel matrix and there by avoid impoverishment of the grain boundaries in chromium as the result of formation of chromium carbides, the intent of the columbium addition being to stabilize the steel against the intergranular corrosion that tends to occur when columbium, titanium or a similar strong carbideforming element is not added and the above-indicated grain-boundary chromium impoverishment is permitted to occur. From the above-indicated columbium range, it will be apparent that in our steel columbium is added in a considerably smaller amount, not more than about double the carbon content, and for a different purpose.

We find that the above-indicated objects of our invention may be achieved by alloying with iron, containing if desired small amounts of incidental impurities in quantities not sufficient to detrimentally affect the properties 3 in any substantial manner and not inconsistent with good steelmaking practice, the following elements, in percent by weight:

Percent Carbon Up to 0.08 Manganese Up to 3.0 Silicon Up to 0.25 Nickel 8 to 20 Chromium 13 to 25 Copper Up to 2.0 One or more of the elements, columbium,

vanadium, titanium or zirconium Boron 0.0005 to 0.025 Nitrogen Up to 0.05 Carbon plus nitrogen Up to 0.08

1 0.01 to 0.25%, col umbium not being present in an amount greater than 0.15%.

A narrower range of composition for our invention encompassing steels having not only desired high strength and good cold forma'bility but also superior resistance to corrosive attack by combustion products of lead-containing fuels, is as follows:

Percent Carbon Up to 0.08 Manganese Up to 3.0 Silicon Up to 0.12 Nickel 8 to 20 Chromium 16 to 25 Copper Up to 2.0 Columbiwm 0.01 to 0.15 Boron 0.0005 to 0.010 Nitrogen Up to 0.05 Carbon plus nitrogen Up to 0.08 Iron and incidental impurities Balance A preferred range of composition for our inventive steel is as follows:

Percent vCarbon 0.01 to 0.05 Manganese Up to 2.0 Silicon About 0.05 to 0.12 Nickel 8 to 20 Chromium 16 to 25 Copper 1.0 to 2.0 Columbium 0.06 to 0.15 Boron 0.002 to 0.009 Nitrogen 0.02 to 0.05 Carbon plus nitrogen Up to 0.06 .Iron and incidental impurities Balance Although alloys from which carbon, manganese silicon, or any of them is entirely absent will exhibit the desirable combination of proper-ties characterizing the steels of our invention, as a practical matter, none of these elements can economically be entirely eliminated. On the other hand, if the specified maxima in respect to carbon and manganese are not observed, strain-cracking will occur when an attempt is made to cold-fonm the steel, and in the case of the specified 0.25% maximum for silicon, the corrosion resistance of the steel at elevated temperatures will also be impaired. Although in a steel with little or no copper the cold-formability becomes impaired when the content of carbon plus nitrogen exceeds about 0.06%, we maintain our broad range of carbon plus nitrogen at 0.08% maximum because such' higher contents can in certain instances be used when copper is added to promote cold-formability. For best results, the silicon content should not exceed 0.12%. For uses involving severe cold forming, it is desirable to limit the maximum manganese content to about 2.0%.

To keep the silicon level low, other deoxidation practices, such 'as vacuum treatment and/or the use of a-luminum, titanium, etc., in appropriate amounts would be employed in the commercial production of our steel.

If less than the specified quantity of nickel is used, the steel tends not to be completely and stably austenitic;

consequently, deformation due to allotropic transformation is encountered and the high-temperature strength is impaired. The use of greater than the specified quantity of nickel is undesirable because of the undesirably high cost of such alloy.

Use of less than the specified amount of chromium impairs the corrosion resistance of the steel. The use of amounts of chromium greater than that indicated above increases the cost of the alloy and increases the quantity of nickel or manganese required to maintain the austenite balance, and this further increases the cost. Moreover, the use of such high amounts of chromium yields a steel or alloy that tends to form sigma phase in its microstructure, and the fabricability of the steel is adversely affected.

With the use of amounts of copper greater than about 2 or 3%, the hot workability of the steel suffers. Copper in the range 1 to 2% is preferred because it promotes cold-formability, and in certain instances when other strengthening elements are substituted for columbium, it improves the creep strength.

If amounts of columbium and boron less than those specified above are used, the elevated-temperature creep strength of the steel is impaired. If amounts of columbium greater than those specified above are used, the steel tends to form delta ferrite, and the matrix tends to become strengthened; this decreases the cold-formability of the steel. With the use of amounts of boron greater than those indicated above, the weldability and cold formability of the steel are adversely affected.

With the use of amounts of nitrogen greater than those indicated above, the yield strength of the steel at ambient temperature tends to become too high, with the result that the metal will not flow during cold-forming, and strain cracking results. Since carbon and nitrogen act in the same way, the specified maximum of carbon plus nitrogen is important and must be observed to avoid strain cracking.

Phosphorus and sulfur may be present as impurities in the amounts usually permitted by commercial standards (about 0.025% maximum for each).

Tantalum may be substituted for columbium, in whole or in part, on the basis of two parts by weight of tantalum for one part of columbium. Other strong carbideforming elements, such as titanium, zirconium, vanadium, molybdenum, tungsten, and possibly hafnium and uranium, may find used, singly or in combination, as substitutes for columbium, in whole or in part, in amounts substantially stoichiometrically equivalent.

Small amounts of aluminum, such as about 0.01 to 0.25%, may also be used in place of columbium, in whole or in part, as a strengthening addition.

So that the general nature of our invention as heretofore disclosed may be more particularly understood, we now disclose certain specific examples thereof and comparative test results.

Eleven different steels were melted as 30-pound induction-melt heats, cast, and forged by the use of conventional methods to bars in. square in cross section. Test specimens of various types were then prepared from the in. square bars. The compositions of these steels were as presented in the following Table I:

TABLE I Steel Bar Mn Si Ni Cr 0. 021 1. 31 0. 01 12. 34 17. 76 0. 034 1. 30 Trace 12. 14 17.82 0. 038 1. 32 0. 01 12. 02 17.53 0. 035 2. 48 0. 01 11.88 18. 0. 043 3. 81 0. 01 11.96 17.90 0. 031 1. 0. 02 11.86 18. 08 0. 024 1. 29 0. 02 11. 96 17. 94 0. 028 1. 0. 02 7. 96 16. 08 0. 032 1. 30 0. 01 16. 20. 0. 102 1. 28 0. 01 11.82 18. 00 0. 035 i 1. 38 Trace 12. 08 17. 76

TABLE I.Continued Tests were conducted to determine the resistance of a the steel to corrosive attack by lead-containing fuel com- Steel C11 Cb B bustion products. Cylindrical specimens about 0.50 in. in diameter and about 0.50 in. long were machined, weighed, g-gg g- Q 'fi 8882 8-853 5 exposed to molten lead oxide at 1675 F. for one hour, Tr'ace N11 @1004 0:042 descaled, and then .reweighed. The test criterion is the g 8% 8-13 8-882 g- 833 8- 82% resultant weight loss per unit surface area of the original (197 01006 01005 01036 specimen. The results of tests with commercial valve 3 8 8 38? 8-88; 88;; steels are presented for comparison. 0.01 0.11 0. 005 0.003 0. 035 0.02 0.10 0.005 0.000 0.108 TABLE III 03 13 0.01 0.11 0. 005 0.04 0.075

Average weight 2 Of the compositions tabulated above, steels, V-l, V-4, Steelv 1 5 (g/m' V-6, V-7, and V9 are within our invention, and the remaining steels are presented for comparison. Steels V-Z and V-3 show the effect of omitting boron and/or columbium. Steel V-5 shows the effect of using greater than 3% of manganese. Steel V-8 shows the effect of using an insufficient amount of nickel. Steels V-10 and V11 demonstrate the effect of using greater than the x specified quantity of carbon plus nitrogen. f3

Creep-test specimens of the above steels were prepared and subjected to tests at 1350 F. under a stress of 6,000 2 0 62 .s.i. for 100 hours. The resultin cree deformation g determined g p Commerclal Valve Steel A 1.20 TABLE H (21Cr4Ni9MnN) Commercial Valve Steel B 2.18 Permanent (2lCr-12N1N) Steel Condition Deformati n, Commercial Valve Steel C 2.13

(19Cr8Ni-3Si) u Commercial Valve Steel D 2.41 git 31000101 1 hour, water quenched (1). (24Cr 5Ni 1M0) v-5 .....(1 0.06 V7 mu 0.11 Additional tests were conducted to determine the effect 33$ 28% 8 3g of silicon content on the resistance of the steel to cor- 1,900 F., 1 houriiifafizilifiziflhij 0.00 rosive attack by lead-containing fuel combustion products.

These results are presentedbelow in Table IV.

TABLE IV Composition, Percent by Weight Average Steel Weight Loss 0 Mn Si Cr Ni Cb B N (gJin?) From the foregoing results, it will be apparent that all the steels tested except steel V-3 exhibit creep resistance substantially equal or superior to that of known commercial valve steels in the hot worked and aged condition, which usually exhibit about 1% permanent deformation after testing for 100 hours at 1350 F. with stresses in the range 5,000 p.s.i. to 8,000 p.s.i.. References herein to the use of strengthening elements in amounts effective to increase substantially the creep strength of the steel will be understood to refer to the addition of columbium and boron, or other elements, in such amount as to lower the permanent deformation, upon testing for 100 hours at 1350 F. and a stress of 6,000 p.s.i., to 0.50% or less, as contrasted with the values of about 1% or greater obtained with unstrengthened completely austenitic stainless steel.

- of the known commercial steels.

The above results indicate that the PbO corrosion resistance of the steel becomes equal to or poorer than that of the above-mentioned commercial steels when the silicon content is about 0.25% or more. It is preferred to use silicon contents of about 0.12% or less, to obtain a substantial improvement in PbO resistance over that Comparison of steel 64-67 with the steels of Table III reveals that no substantial additional improvement is obtained by using silicon contents lower than about 0.06%, and for this reason we prefer to use silicon contents in the range about 0.06 to 0.12%.

That the variations in carbon and boron contents in the above Table IV have no effect on the PbO corrosion resistance is apparent from the following Table V.

TABLE V Composition, Weight Percent Average Steel Weight Loss Mn S1 Ni Cr Cu Cb B N (g./ln.

V- 0. 024 1.29 0.02 11. 96 17. 94 1. 97 O. 09 0.005 0.003 0.72 V-2l 0. 018 1. 36 0. 02 12. 10 18. 26 1. 92 0. l4 0. 009 0. 007 0. 74 V-1. 0. 021 l. 31 0. 01 12. 34 17. 76 0. 02 0. 11 0. 004 0. 003 0. 62 V-lOv 0. 102 l. 28 0. 01 11. 82 18. 00 0. 02 0. 10 0. 005 0. 006 0.62

The following Tables VI and VII show that boron contents over about 0.009% detrimentally affect the cold formability. Table VI presents the results with steels different boron contents and Table VII shows the criterion used in arriving at the cold-formability rating.

hour at 1800 F. and then Water quenching in the case of steels V-l through V9, inclusive, and annealing at 1900 F. for one hour and then water quenching in the case of steels V-10 and V11, were used. The results are presented in the following Table VIII:

TABLE VI Steel 0 Mn Si Ni Cr Cu Cb B N v-7 0.024 1. 29 0.02 11.96 17.94 1.97 0.09 0. 005 0. 003 v-21 0.018 1.36 0.02 12.10 18.26 1.92 0.14 0. 009 0.007 V-22 0.009 1. 43 0.08 12.18 18.20 1.87. 0.12 0.020 0.008

Relative cold- TABLE vm Steel: formability rating V-7 OOd- 0.2% Yield Tensile Elongation Reduction V 21 i Steel Strength, Strength, in 1.4in., of Area, V 22 y p .p.s.i. p.s.i..v Percent Percent 27, 000 78,000 7s Specimens for the cold-formabihty test (0.500 inch 26,000 79,000 70 diameter by 2 inches long) were machined from annealed 3g $8 samples of steels V-7, V-21 and V22. Three specimens 28,000 80,000 71 70 of each steel Were tested by being placed in a die in such 32 888 888 2g 3g manner that a 1-inch free length was subjected todeforma- 251000 1251000 35 71 tion. A No. 1 /2 National Maxipress at a speed ap- 838 883 proximating 1200 inches per minute was used to effect 60,000 81,000 70 78 deformation. The upset end of each specimen was sub jected to a reduction in height of the order of 87%, determined by measuring the thickness of the upset head and assuming that its original length was 1 inch. The head-diameter ratio, that is, the diameter of the upset head divided by its original diameter ranged from 2.6 to 2.7. The specimens were then visually examined for shear cracks, and a rating was assigned in accordance with the criterion in Table VII below.

Conventional room-temperature tension tests were conducted with the above-indicated eleven steels. 0.505 in. in diameter, heat treated by annealing for one Specimens 9 In the foregoing table, steels V-l, V-4, V6, V-7, and V9 represent steels of our invention, and the remaining steels are presented for comparison. All the steels of the invention were free of tendency for cracking when upset 68%, and all except steels V-l and V-9 were free of tendency for cracking when upset 80%. Of the remaining steels, V2 and V3 exhibited poor creep strength, V-S had a tendency to stress cracking at the 80% upset level because of its manganese content, V8 was not completely austenitic, and V10 and Vll developed stress 10 cracking during cold forming because of their contents of carbon and nitrogen. This demonstrates that the full advantages of the invention are obtained only with steels falling within fairly narrow and critical ranges.

Additional test data show the eifect of using strengthening elements other than columbium and the effect of copper. We have made additions of vanadium, zirconium, titanium, and chromium to our steels having the nominal composition of Permanent Deformation at 1,350 F. under 6,000 p.s.i. for 100 hr., percent Steel Code Percent The composition of the steels tested is set forth in the Carbon Up to 0,08 following Table X: Manganese Up to 3.0

TABLE X Steel 0 Mn Si Ni Cr Cu B N Other 0 08 11.84 18.04 1.81 0.000 0. 003 015v 0 02 12.08 18.24 1.79 0. 000 0. 003 0.030 Zr 0 03 11.96 18.12 1.87 0.005 0. 003 0.09 Ti 0 01 11.90 18.32 1.84 0.006 0. 003 0.03 Al 0 02 11.90 17. 94 1.97 0. 005 0. 003 0.09 Cb The steels were tested for cold formability, lead-oxide Silicon Up to 0.12 corrosion resistance, and permanent creep deformation Nlckel 8 to (strain) as described above. The following Table XI is Chromlum. 16 to a summary of the pertinent results. Copper Up to 2.0

TABLE XI Average Wt. Permanent Significant Relative Cold Loss in PbO Deformation Steel Code Compositional Formability Test at 1,675 F. at 1,350 F.

Variable, Rating for 1 hr. under 6,000 percent (g./in. p.s.i. for 100 hr., percent It is evident that V, Zr, Ti, and Al all have beneficial Columbium 0.01 to 0.15 effects similar to those obtained with Cb; therefore, similar Boron 0.0005 to 0.010 amounts of V, Zr, Ti, and Al can be used in our steels to Nitrogen Up to 0.05 achieve the cold-formable exhaust valve material that is Carbon plus nitrogen Up to 0.08 the object of our invention. 60 Iron and incidental impurities Balance Data showing the beneficial eifect of copper on the stretch resistance (creep strength) of our steels are presented in the following Table XII.

TABLE XII Steel Code 0 Mn S1 Ni Cr Cu B N Other 0. 06 1. 43 0. 02 12. 08 18. 24 O. 003 0.036 Zr 0. 031 1. 0. 03 11. 94 18. 43 0. 004 0.10 Al 0. 05 1. 39 0. 02 11. 98 18. 26 0. 003 0.09 Al 1 l 2. In the method of utilizing an internal combustion engine having valves therein, the improvement which comprises using a valve of an alloy consisting essentially of, in Weight percent,

Percent Carbon 0.01 to 0.05 Manganese Up to 2.0 Silicon About 0.05 to 0.12 Nickel 8 to 20 Chromium 16 to 25 Copper 1.0 to 2.0 Columbiurn 0.06 to 0.15 Boron 0.002 to 0.009 Nitrogen 0.02 to 0.05 Carbon plus nitrogen Up to 0.06

Iron and incidental impurities Balance 1 2 References Cited by the Examiner UNITED STATES PATENTS 2,150,901 3/1939 Arness 75128.5 2,496,248 5/ 1948 Jennings 7512'8.0 2,624,671 1/1951 Binder 75128.5 2,784,125 3/1957 Clarke 75128.6 3,107,997 10/1963 Kozlik 75128.4 3,139,337 6/1964 Boyle et al. 75--128.6

OTHER REFERENCES Salvaggi and Ter-kovich, Transactions of American Society for Metals, volume 49, Preprin-t No. 33.

I-IYLAND BIZOT, Primary Examiner.

DAVID L. REOK, Examiner. H. W. TAR-RING, P. WEINSTEIN, Assistant Examiners. 

1. IN THE METHOD OF UTILIZING AN INTERNAL-COMBUSTION ENGINE HAVING VALVES THEREIN, THE IMPROVEMENT WHICH COMPRISES USING A VALVE OF AN ALLOY CONSISTING ESSENTIALLY OF, IN WEIGHT PERCENT, 